Semiconductor devices are thin structures that are fabricated over the surface of a substrate by adding and removing material in several steps. Various layers are typically built up, with each layer having a composition and a form selected in view of the device design. The process may include steps to alter the properties of certain deposited materials, e.g., through ion implantation or annealing. In addition, chemical-mechanical planarization may be used to smoothen out the layers as they are built up.
The patterning of deposited material usually involves lithography. First, a uniform layer of material is deposited on the entire surface of the substrate. Unwanted material is then removed by an etching process, such as wet etching or dry etching (e.g., plasma etching or ion milling). Lithography permits certain portions of the layer to be removed while others remain. A typical lithographic process involves coating a layer with a photo-sensitive resist, selectively exposing the resist to a specific wavelength of light using focusing optics and a mask, chemically washing away the exposed (or alternatively, unexposed) portions of the resist, etching away parts of one or more layers, and then removing the remaining resist before proceeding to the next step. These steps involve the use of expensive equipment and can be time consuming.
Shadow masking is an alternative patterning method that does not rely on a lithographic process. Instead, a mask of a desired pattern is placed between the deposition source and the substrate, so that only material that passes through openings in the mask is deposited on the substrate. The mask is ideally positioned as close to the substrate as possible, so that the deposited pattern does not become “defocused” on the substrate; rather, the resulting deposited pattern is very nearly an exact copy of the mask. With shadow masking, the achievable feature sizes are limited by mask manufacturing capabilities, and precision mechanical alignment is required between the mask and the substrate. Nevertheless, shadow masking permits the rapid prototyping of devices, is significantly cheaper (since expensive processing equipment, such as steppers and etchers are not needed), generally involves fewer process steps than lithography, and does not require the use of harsh chemicals.